Extraction of bitumen from mined oil sands with hydrocarbon solvents uses little or no water, generates no wet tailings, and can achieve higher bitumen recovery than the existing Clark hot water extraction process or its variants. A problem which prevents solvent extraction processes from commercialization is the ineffective solvent recovery from spent oil sand solids. The problem becomes more challenging in large-scale oil sands operation, typically 8000 t/h of oil sands throughput per production train.
The prior art describes various methods for solvent recovery including, for example, water washing (see for example, U.S. Pat. Nos. 4,311,561 and 4,968,412); and saturated steam stripping using a filter bed, vertical column, rotary drum, or other vessels (see for example, U.S. Pat. Nos. 3,475,318; 4,189,376; 4,422,901; 4,448,667; 4,460,452; 4,719,008 and 4,722,782; and Canadian Patent Application No. 2,734,067). None of the prior art addresses the problem of solvent recovery from spent oil sand solids which contain 3-10 wt % water and 3-10 wt % solvent. Water washing generates undesirable wet tailings that negate the key benefit of using solvent extraction. Saturated steam stripping supplies heat to vaporize solvent by condensing steam and adding water to the solids. However, significant amounts of solvent remain trapped in solid lumps, which are apparently formed by water binding the solids through capillary pressure. The occluded solvent in solid lumps is difficult to recover. In commercial oil sand processing, solvent-laden spent solids are fed into a vertical desolventizer heated with steam jackets and live steam, and having multiple horizontal trays equipped with sweep arms for agitating solids (Williams, 2005). Such a complex design would be impractical and uneconomical in the oil sands industry.
Indirect drying processes in a rotary kiln, rotary dryer, or steam tuber dryer have been described (see for example, Canadian Patent Application No. 2,012,563; US Patent Application Publication No. 2009/0294332; U.S. Pat. No. 4,139,450). Such processes remove both water and solvent, thus disintegrate solid lumps during drying, thereby generating dry solids meeting environmental regulations for solvent content. The recovered solvent vapor and steam mixed with small amount of inert gas used as sweep gas can be condensed and recycled. However, the heat transfer coefficient for oil sand solids through heat conduction is at or less than about 60 W/m2K, which makes the total heating surface area required for drying spent oil sand solids at 8000 t/h prohibitively large.
Direct drying using hot inert gas or flue gas in a fluid bed dryer, multi-hearth furnace and turbo dryer have been described (see for example, U.S. Pat. Nos. 4,347,118 and 5,534,136; Canadian Patent Application No. 2,715,301). Direct drying processes would produce similar low-solvent dry solids as when using indirect drying processes. Compared to indirect drying, direct drying allows more efficient heat exchanging and is amenable to handle a large solids throughput. However, hot flue gas or inert gas typically dilutes the solvent vapor, impeding solvent recovery.
Superheated solvent vapor drying has been described (see for example, U.S. Pat. Nos. 4,347,118 and 4,539,093). Since the heating medium is a condensable vapor, stripped solvent can be readily recovered; however, superheated solvent vapor is highly flammable and tends to coke and foul equipment. The same problem is present if any commercial superheated steam dryer is used directly for spent oil sand solids. These dryers recycle and superheat part of the produced vapors (Monceaux et al., 2009), which contain large amount of solvent vapors.
U.S. Pat. No. 4,571,294 describes a process using superheated steam to recover solvent. The used steam and produced vapors are condensed without being recycled, thus eliminating any fire hazard. The process applies to pelletized diatomaceous earth in which solvent would not be trapped within the pellets. The heat duty of solvent stripping is light. Therefore, a short-residence-time vertical column stripper was proposed in this invention. This process would be inadequate if it is used directly for spent oil sand solids, which trap solvent within the solid lumps and require significant drying (removal of water) for solvent recovery. A study showed that the water concentration must be decreased to 2 wt % or less (see for example, Canadian Patent Application No. 2,724,806) to reduce the solvent content in spent oil sand solids to an acceptable level.
Accordingly, there is a need for an improved method of recovering solvent from spent oil sand solids.